Adsorption Characteristics between Ti Atoms of TiO2(100) and Corrosive Species of CO2-H2S-Cl− System in Oil and Gas Fields

The service environment of OCTG (Oil Country Tubular Goods) in oil and gas fields is becoming more and more severe due to the strong affinity between ions or atoms of corrosive species coming from solutions and metal ions or atoms on metals. While it is difficult for traditional technologies to accurately analyze the corrosion characteristics of OCTG in CO2-H2S-Cl− systems, it is necessary to study the corrosion-resistant behavior of TC4 (Ti-6Al-4V) alloys based on an atomic or molecular scale. In this paper, the thermodynamic characteristics of the TiO2(100) surface of TC4 alloys in the CO2-H2S-Cl− system were simulated and analyzed by first principles, and the corrosion electrochemical technologies were used to verify the simulation results. The results indicated that all of the best adsorption positions of corrosive ions (Cl−, HS−, S2−, HCO3−, and CO32−) on TiO2(100) surfaces were bridge sites. A forceful charge interaction existed between Cl, S, and O atoms in Cl−, HS−, S2−, HCO3−, CO32−, and Ti atoms in TiO2(100) surfaces after adsorption in a stable state. The charge was transferred from near Ti atoms in TiO2 to near Cl, S, and O atoms in Cl−, HS−, S2−, HCO3−, and CO32−. Electronic orbital hybridization occurred between 3p5 of Cl, 3p4 of S, 2p4 of O, and 3d2 of Ti, which was chemical adsorption. The effect strength of five corrosive ions on the stability of TiO2 passivation film was S2− > CO32− > Cl− > HS− > HCO3−. In addition, the corrosion current density of TC4 alloy in different solutions containing saturated CO2 was as follows: NaCl + Na2S + Na2CO3 > NaCl + Na2S > NaCl + Na2CO3 > NaCl. At the same time, the trends of Rs (solution transfer resistance), Rct (charge transfer resistance), and Rc (ion adsorption double layer resistance) were opposite to the corrosion current density. The corrosion resistance of TiO2 passivation film to corrosive species was weakened owing to the synergistic effect of corrosive species. Severe corrosion resulted, especially pitting corrosion, which further proved the simulation results mentioned above. Thus, this outcome provides the theoretical support to reveal the corrosion resistance mechanism of OCTG and to develop novel corrosion inhibitors in CO2-H2S-Cl− environments.


Introduction
Corrosion has been considered as one of the major social problems in pipelines and industries using such materials since the early industrial revolution. A large number of accidents occur frequently, which leads to a greater threat to the safe production of oil and gas [1]. The annual cost of corrosion in China is about CNY 2.3 trillion, accounting for 3.3% of GDP [2].
In recent years, with the development of deep and ultra-deep wells to meet the social demand for energy, the working environment of tubing and casing is becoming more and more complex. In addition to the stringent service conditions, metal OCTG are inevitably subjected to different degrees of corrosion, and the working properties of OCTG have decreased. For example, the presence of H 2 S leads to severe localized corrosion, as well as cracks caused by stress and hydrogen [3]. In some special working conditions, CO 2 and H 2 S exist at the same time [4], which greatly deteriorates the service environment of OCTG [5]. The high temperature, pressure, acid gas content, and Cl − concentration of oil and gas wells increase the requirements for corrosion-resistant OCTG.
The TC4 titanium alloy (Ti-6Al-4V) is now considered to be the ideal material applied in oil and gas fields, accounting for about half of the market share of titanium alloys currently used in the world [6]. A dense TiO 2 oxide film with a thickness of 4~6 nm of TC4 will be spontaneously formed at room temperature [7], which can effectively prevent the matrix in a solution from being corroded by corrosive ions (such as H + , Cl − , etc.) [8]. However, the film is not always able to maintain its integrity; it is very likely to be destroyed in some medium containing some corrosive species, resulting in serious corrosion of the titanium alloy matrix [9]. It is difficult for traditional technologies to accurately analyze the corrosion characteristics of OCTG in CO 2 -H 2 S-Cl − systems.
Therefore, the first-principles calculation software (Materials Studio) on account of DFT (Density Functional Theory) was selected to research the interface characteristics between the corrosive species and TiO 2 passivation film on the surface of TC4 alloys in CO 2 -H 2 S-Cl − systems containing Cl − , HS − , S 2− , HCO 3 − , and CO 3 2− based on an atomic or molecular scale. Additionally, the corrosion characteristics of the TC4 alloy in NaCl, NaCl + Na 2 CO 3 , NaCl + Na 2 S, and NaCl + Na 2 S + Na 2 CO 3 solutions containing saturated CO 2 were carried out by the electrochemical technologies to verify the simulation results above.

Modeling
TiO 2 passivation film on titanium alloy surfaces has three crystal structures: rutile, anatase, and brookite [10]. Figure 1 shows the Raman spectra of TiO 2 film on TC4 alloy, and Table 1 shows the frequency shift positions of Raman spectral characteristics of three crystalline TiO 2 . The four peaks, 145.53 cm −1 , 241.76 cm −1 , 612.53 cm −1 , and 824.16 cm −1 in Figure 1, are consistent with the corresponding peak value of the rutile TiO 2 in Table 1. Some scholars found that the composition of titanium alloy passivation film was rutile TiO 2 [11]. Therefore, rutile TiO 2 was selected as the research object in this paper.  There are (110), (100), and (001) low index surfaces in Rutile phase TiO 2 . The characteristics of the various ions on TiO 2 (110) surfaces have been studied, including our previous research [2], but few reports were focused on the adsorption of TiO 2 (100) and TiO 2 (001) surfaces. Furthermore, compared with TiO 2 (001) surfaces, TiO 2 (100) surfaces present a higher possibility of stable existence at high temperatures [12]. Therefore, the adsorption properties of various corrosive ions (Cl − , HS − , S 2− , HCO 3 − , and CO 3 2− ) on rutile TiO 2 (100) surfaces were studied.
The CASTEP in Material Studio, the first-principles computing software, was used to conduct geometric optimization for all adsorption configurations [13]. According to the setting requirements of the CASTEP module, a 2 × 3 × 1 three-dimensional supercell structure with periodic boundary conditions was established for the rutile TiO 2 (100) surface. In addition, a vacuum area with a thickness of 20 Å was added between the two plates to prevent interaction between them [14]. Figure 2 reveals the boundary surface models of various corrosive ions (Cl − , HS − , S 2− , HCO 3 − , and CO 3 2− ) at different adsorption sites (top, bridge, and hole) on a TiO 2 (100) surface.
Materials 2023, 16, x FOR PEER REVIEW  3 of 13 surface. In addition, a vacuum area with a thickness of 20 Å was added between the two plates to prevent interaction between them [14]. Figure 2 reveals the boundary surface models of various corrosive ions (Cl − , HS − , S 2− , HCO3 − , and CO3 2− ) at different adsorption sites (top, bridge, and hole) on a TiO2(100) surface.

Computing Method
Using the PBE functional of GGA, the pseudopotentials were constructed using the plane wave ultrasoft pseudopotential SCF [1,12,15], where the truncation energy of the plane wave was set as 400 eV, the convergence accuracy in the iteration process was 2 × 10 −6 eV/atom, the self-consistent iteration was 300 times, the force converge was 0.03 eV/atom, the tolerance deviation was not higher than 0.005, the stress deviation was under 0.08 GPa, and the k-points value was 2 × 3 × 1 in the Brillouin zone.

Computing Method
Using the PBE functional of GGA, the pseudopotentials were constructed using the plane wave ultrasoft pseudopotential SCF [1,12,15], where the truncation energy of the plane wave was set as 400 eV, the convergence accuracy in the iteration process was 2 × 10 −6 eV/atom, the self-consistent iteration was 300 times, the force converge was 0.03 eV/atom, the tolerance deviation was not higher than 0.005, the stress deviation was under 0.08 GPa, and the k-points value was 2 × 3 × 1 in the Brillouin zone.

Preparation of Experimental Materials
The electrochemical test sample was TC4 titanium alloy, which was ø10 mm × 3 mm. A wire was welded to one end of the sample and tested for conductivity with a multimeter to verify whether the wire was welded correctly. The surface at the other end of the sample was the electrochemical test surface. The surface other than the electrochemical test surface was glued and stamped with epoxy resin AB glue and then polished with sandpaper with mesh sizes of 400 # , 800 # , 1200 # , 1500 # , and 2000 # . For the purpose of reaching the test requirements for sample roughness, the sample surface was polished to 2000 # , cleaned with distilled water, degreased with acetone, dehydrated and desiccated with alcohol, and dried with cold air for later use.

Experimental Methods and Equipment
The electrochemical test was carried out by Princeton P4000 electrochemical workstation, in which the working electrode was TC4 alloy, the reference electrode was polytetrafluoro silver chloride, and the auxiliary electrode was a platinum electrode. Before the electrochemical test, high-purity nitrogen was used to deoxygenate the required corrosive medium for 1 h., and the temperature was heated up to the preset temperature (80 • C). The electrochemical test was performed when the entire test system reached stability, and each experiment was performed three times.
The working electrode was pre-polarized at a voltage set value of −1200 mV for 3 min in advance of the electrochemical test. After the oxide film spontaneously took shape on the surface of the sample in the air and was eliminated, the working electrode was put in the prepared medium and stood for 30 min to form new film. The test frequency was set to 10 −2 HZ~10 5 HZ, the measured signal amplitude was 10 mV sine wave, and the number of points was 50. The scanning rat was set as 0.3333 mV/s, and the potential was −1000 mV~+1600 mV.

Stable Adsorption Model
To simulate the species in the CO 2 -H 2 S-Cl − environment (CO 2 +H 2 O→H 2 CO 3 , H 2 CO 3 →H + +HCO 3 − , HCO 3 − →H + +CO 3 2− , H 2 S→H + +HS − , HS − →H + +S 2− ), the final energy of five corrosive ions (Cl − , HS − , S 2− , HCO 3 − , and CO 3 2− ) at different adsorption sites on TiO 2 (100) surface after geometric optimization is shown in Table 2. By comparison, it was found that the energy of each corrosive ion was the lowest at the bridge site of the TiO 2 (100) surface. If the energy of the adsorption system were more negative, its structure would be more stable [16]. Therefore, it can be determined that all of the best adsorption sites of Cl − , HS − , S 2− , HCO 3 − , and CO 3 2− on the TiO 2 (100) surface were bridge sites. The final energy of each corrosive ion at the bridge site of the TiO 2 (100) surface was in the following order: ions, leading to the TiO 2 passivation film suffering from stronger corrosion. It could be seen that the stability of the TiO 2 in the environment containing corrosive species was the following: S 2− < CO 3 2− < Cl − < HS − < HCO 3 − . That is, TiO 2 film on the surface of TC4 alloy is more easily damaged in the mediums containing S 2− than in CO 3 2− , Cl − , HS − , and HCO 3 − .

Charge Density
, which are in accordance with our previous research [2]. The metal surface with a higher charge density value is more likely to be corroded by the corrosive ions, leading to the TiO2 passivation film suffering from stronger corrosion. It could be seen   Figure 4 shows the charge density of the corrosive ions (Cl − , HS − , S 2− , HCO 3 − , and CO 3 2− ) at the bridge site of the TiO 2 (100) under the stable state adsorption. It could be seen that a very distinct charge transfer appearance was presented between Cl, S, O, and Ti atoms which was in the Cl − , HS − and S 2− , HCO 3 − and CO 3 2− , and TiO 2 (100) surfaces, respectively. Charge segregation and electronegativity decreased near Cl, S, and O atoms, while charge dissipation and electronegativity increased near the Ti atom in TiO 2 [17]. Therefore, the interface binding energy between Cl, S, O, and Ti atoms was in the Cl − , HS − and S 2− , HCO 3 − and CO 3 2− , and TiO 2 (100) surfaces, respectively. Finally, the specific charge transfer process moved from the Ti atom on the TiO 2 (100) surface to Cl, S, and O atoms. following: S 2− < CO3 2− < Cl − < HS − < HCO3 − . That is, TiO2 film on the surface of TC4 alloy is more easily damaged in the mediums containing S 2− than in CO3 2− , Cl − , HS − , and HCO3 − .  Figure 4 shows the charge density of the corrosive ions (Cl − , HS − , S 2− , HCO3 − , and CO3 2− ) at the bridge site of the TiO2(100) under the stable state adsorption. It could be seen that a very distinct charge transfer appearance was presented between Cl, S, O, and Ti atoms which was in the Cl − , HS − and S 2− , HCO3 − and CO3 2− , and TiO2(100) surfaces, respectively. Charge segregation and electronegativity decreased near Cl, S, and O atoms, while charge dissipation and electronegativity increased near the Ti atom in TiO2 [17]. Therefore, the interface binding energy between Cl, S, O, and Ti atoms was in the Cl − , HS − and S 2− , HCO3 − and CO3 2− , and TiO2(100) surfaces, respectively. Finally, the specific charge transfer process moved from the Ti atom on the TiO2(100) surface to Cl, S, and O atoms.    Figure 5 shows PDOS (Projected Density of States) diagrams of five corrosive ions at the bridge site of the TiO 2 (100) surface, which can be calculated to investigate the characteristics of various ions on the TiO 2 (100) surface deeply [18]. It could be seen that a certain extent of the charge interaction existed between Cl, S, O, and Ti atoms, indicating that the adsorption process was chemical adsorption [19]. The charge interaction and interfacial bonding were primarily made of hybrid orbitals between 3d 2 of the Ti atoms and 3p 5   that the adsorption process was chemical adsorption [19]. The charge interaction and interfacial bonding were primarily made of hybrid orbitals between 3d 2 of the Ti atoms and 3p 5 of Cl, 3p 4 of S,and 2p 4 of O.

Binding Energy
The corrosiveness of each corrosive ion to the matrix can be ensured through the interface binding energy, which was calculated as follows [20]: Et is the total energy of whole model after geometry optimization; E1 is the energy after geometric optimization of TiO2(100); E2 is the energy after geometric optimization of each corrosive ion. Table 4 displays the final energy between Cl − , HS − , S 2− , HCO3 − , CO3 2− , and TiO2(100) after geometric optimization. According to Tables 2 and 4, combined with Formula (1), the interface binding energies of various corrosive ions (Cl − , HS − , S 2− , HCO3 − , and CO3 2− ) at the bridge site of the TiO2(100) surface were obtained, as shown in Table 5. It could be seen that when HCO3 − was adsorbed on the TiO2(100) surface, the entire adsorption system had low energy. Compared with Cl − , HS − , HCO3 − , and CO3 2− , the interface between S 2− and TiO2(100) was easier to bond and react, indicating that S 2− had a stronger adsorption

Binding Energy
The corrosiveness of each corrosive ion to the matrix can be ensured through the interface binding energy, which was calculated as follows [20]: E t is the total energy of whole model after geometry optimization; E 1 is the energy after geometric optimization of TiO 2 (100); E 2 is the energy after geometric optimization of each corrosive ion. Table 4 displays the final energy between Cl − , HS − , S 2− , HCO 3 − , CO 3 2− , and TiO 2 (100) after geometric optimization. According to Tables 2 and 4, combined with Formula (1), the interface binding energies of various corrosive ions (Cl − , HS − , S 2− , HCO 3 − , and CO 3 2− ) at the bridge site of the TiO 2 (100) surface were obtained, as shown in Table 5. It could be seen that when HCO 3 − was adsorbed on the TiO 2 (100) surface, the entire adsorption system had low energy. Compared with Cl − , HS − , HCO 3 − , and CO 3 2− , the interface between S 2− and TiO 2 (100) was easier to bond and react, indicating that S 2− had a stronger adsorption capacity on TiO 2 . Therefore, TiO 2 has poor stability in the environment containing S 2− . The steadier the interface model is, the smaller interface binding energy is [20], so the film stability of TiO 2 in the solutions containing Cl − , HS − , S 2− , HCO 3 − , and CO 3 2− was S 2− < CO 3 2− < Cl − < HS − < HCO 3 − , which is consistent with the charge density results mentioned above. −69171.5619281723 Table 5. Interface binding energies of five ions on TiO 2 (100) surface at bridge sites.

Alternating-Current Impedance
The alternating-current impedances of TC4 alloy in NaCl, NaCl + Na 2 CO 3 , NaCl + Na 2 S and NaCl + Na 2 S + Na 2 CO 3 solutions containing saturated CO 2 are shown in Figure 6. It could be seen that the radius of the capacitive arc of TC4 alloy in four corrosive solution was NaCl > NaCl + Na 2 CO 3 > NaCl + Na 2 S > NaCl + Na 2 S + Na 2 CO 3 . The radius of electrochemical Nyquist impedance spectroscopy can determine the corrosion resistance of materials; the larger the radius of the electrochemical Nyquist impedance spectrum is, the stronger the corrosion resistance of materials to local corrosion is [21]. Therefore, the corrosiveness of four corrosive solutions to TC4 alloy was NaCl + Na 2 S + Na 2 CO 3 > NaCl + Na 2 S > NaCl + Na 2 CO 3 > NaCl. The equivalent circuit was shown in Figure 7. It can be seen that Cdl (double layer capacitance) and Cc (ion adsorption double layer capacitance on the electrode surface) increased, and that both Rct (charge transfer resistance) and Rc (ion adsorption double layer resistance) decreased, concluding that the TC4 alloy has poor corrosion resistance [22]. The equivalent circuit was shown in Figure 7. It can be seen that C dl (double layer capacitance) and C c (ion adsorption double layer capacitance on the electrode surface) increased, and that both R ct (charge transfer resistance) and R c (ion adsorption double layer resistance) decreased, concluding that the TC4 alloy has poor corrosion resistance [22].
The equivalent circuit was shown in Figure 7. It ca capacitance) and Cc (ion adsorption double layer capac increased, and that both Rct (charge transfer resistance) layer resistance) decreased, concluding that the TC4 allo [22]. As seen in Table 6, when there was only NaCl in th adsorption double layer capacitance on the electrode su value of double layer capacitance was 4.925 × 10 −6 , the Rc Rct value was 3.135 × 10 4 Ω·cm 2 . With the addition of CO3 2 increased to varying degrees, while the values of Rc and S 2− exited together, the corresponding electrochemical p sion resistance of the TC4 alloy to four solutions is NaCl > NaCl + Na2S + Na2CO3, which is consistent with the abov  As seen in Table 6, when there was only NaCl in the electrolyte, the C c value of ion adsorption double layer capacitance on the electrode surface was 3.617 × 10 −7 , the C dl value of double layer capacitance was 4.925 × 10 −6 , the R c value was 1565 Ω·cm 2 , and the R ct value was 3.135 × 10 4 Ω·cm 2 . With the addition of CO 3 2− and S 2− , the values of C c and C dl increased to varying degrees, while the values of R c and R ct decreased. When CO 3 2− and S 2− exited together, the corresponding electrochemical parameters increased. The corrosion resistance of the TC4 alloy to four solutions is NaCl > NaCl + Na 2 CO 3 > NaCl + Na 2 S > NaCl + Na 2 S + Na 2 CO 3 , which is consistent with the above numerical simulation results.  Figure 8 displays the polarization curves of the TC4 titanium alloy in four corrosive media (NaCl, NaCl + Na 2 CO 3 , NaCl + Na 2 S, NaCl + Na 2 S + Na 2 CO 3 ). Table 7 shows the fitting results. The i corr (self-corrosion current density) was 1.689 × 10 −4 mA/cm 2 , and the E corr (self-corrosion potential) of TC4 alloy in NaCl solution containing saturated CO 2 was −578 mV. With the addition of CO 3 2− or/and S 2− , the E corr of the electrode decreased, and i corr increased. The E corr can reflect the tendency of corrosion [23], and the i corr represents the speed of corrosion rate. The value of the i corr is larger, indicating that the corrosion rate is more rapid [24]. It could be seen that the TC4 titanium alloy showed excellent corrosion resistance in a corrosive solution containing only NaCl. In a NaCl + Na 2 CO 3 solution, the resistance of the TC4 alloy decreased. While in the NaCl + Na 2 S + Na 2 CO 3 solution, the TC4 alloy suffered from the most severe corrosion. This finding is consistent with the above alternating-current impedance results and numerical simulation results.
The results of the electrochemical experiments mentioned above also are in good accordance with the previous research in a 35% NaCl + 0.4% Na 2 S solution at 80 • C [25], as shown in Table 8.
TC4 alloy suffered from the most severe corrosion. This finding is consi above alternating-current impedance results and numerical simulation resu The results of the electrochemical experiments mentioned above als accordance with the previous research in a 35% NaCl + 0.4% Na2S solution as shown in Table 8.

Conclusions
(1) All of the most suitable adsorption sites of corrosive ions (Cl − , HS − , S 2− , HCO 3 − , and CO 3 2− ) on the TiO 2 (100) surface were bridge sites, then hole sites and top sites. (2) A forceful charge interaction occurred between Cl, S, O, and Ti atoms. The charge was transferred from near the Ti atoms in the TiO 2 (100) surface to near Cl, Ss, and O atoms in Cl − , HS − , S 2− , HCO 3 − , and CO 3 2− , respectively. Interface binding energy was primarily formed by electronic orbital hybridization between 3p 5 of Cl, 3p 4 of S, 2p 4 of O, and 3d 2 of Ti, and they were chemical adsorption.
(3) Interface binding energy between five corrosive species and the TiO 2 (100) was as follows: S 2− > CO 3 2− > Cl − > HS − > HCO 3 − . (4) With the addition of CO 3 2− and S 2− , local corrosion of the TC4 alloy in an NaCl solution containing saturated CO 2 increased, especially the synergistic effect between Cl − , CO 3 2− , and/or S 2− , which made the corrosion electrochemical parameters of TC4 alloy change by two orders of magnitude.